1. Field of the Invention
The present disclosure relates to display apparatuses, driving devices for light emitting devices and image forming apparatuses, and it particularly relates to display apparatuses including an organic electroluminescent (EL) element and a drive circuit therefor.
2. Description of the Related Art
In recent years, demands for higher resolutions of active matrix display apparatuses including light emitting devices such as organic EL elements have risen, and the area that may be assigned to one pixel has been reduced therefor. The area to be assigned to a pixel has a light emitting device and a drive circuit which drives the light emitting device. Thus, the reduced area that may be assigned to one pixel may reduce the area to be assigned to the drive circuit. This may require the drive circuit configured with as few circuit elements as possible.
The drive circuit includes a transistor (hereinafter, called a drive transistor) which controls the amount of current to be supplied to the light emitting device in accordance with an image signal. However, transistors vary in their electric characteristics due to their manufacturing processes. Particularly, thin film transistors having been used widely vary largely in their threshold values and/or mobilities. This may cause different amounts of current to be supplied to the light emitting devices even in response to a same image signal in a display apparatus employing thin film transistors as the drive transistors, resulting in display unevenness and thus deteriorating its display quality.
According to Japanese Patent Laid-open No. 2003-140613, the current fed to a light emitting device is amplified by a current mirror circuit and is fed back to a data-side drive circuit, and the feedback current and a reference current having luminance information are compared. Then, the current to be fed to the light emitting device is controlled to be a drive current having a desirable luminance.
Japanese Patent Laid-open No. 2003-271095 discloses a drive circuit which generates current independent of the threshold voltage of a drive transistor. Before data voltage is written, a current path between the drive transistor and the light emitting device is blocked, and a short circuit is created between its gate and drain. Thus, the drain current of the drive transistor discharges the gate-source capacitor and thus gradually reduces the gate-source voltage. When the gate-source voltage gets to be equal to the threshold voltage of the drive transistor, the drain current is equal to 0. This holds the threshold voltage in the capacitor. Such an operation in which current fed to a transistor brings a gate-source voltage to a threshold voltage of the transistor is called auto-zero.
In order to write a data voltage to a drive circuit with auto-zero when a gate-source capacitor of the drive transistor holds a threshold voltage, the voltage of the data line is changed from a reference voltage to the data voltage, and the voltage change on the data line is transmitted to a gate of the drive transistor through a different capacitor connected to between the gate and the data line. This changes the voltage across the gate-source capacitor from the threshold voltage by the amount equal to the voltage proportional to the voltage change on the data line. The gate-source voltage after the change exhibits a value including the addition of the change proportional to the data voltage to the threshold voltage. This provides a drain current independent of the threshold voltage.
According to Japanese Patent Laid-open No. 2003-140613, a data-side drive circuit is provided for each data line and is shared by a plurality of pixels, eliminating the necessity for a data-side drive circuit for each pixel. Instead, it may require the current (monitor current) acquired by amplifying the current fed to each light emitting device to be fed back to a data-side drive circuit through a data line. When the monitor current is minute current, it is influenced by parasitic capacitance of the data line, which may require time for detection and/or correction the minute current. The drive circuit may possibly become bloated when higher amplification efficiency by a current mirror circuit for higher monitor current are attempted to obtain.
A pixel circuit performing an auto-zero operation according to Japanese Patent Laid-open No. 2003-271095 has two of a gate-source capacitor and a gate-data line capacitor. This increases the area of occupation of the capacitors on the display substrate and reduces the pixel size, making it difficult to provide a high-definition display apparatus.